Exhaust-driven turbocharger-type superchargers that perform supercharging using flow strength of an exhaust stream are frequently used as superchargers to improve the intake efficiency of an internal combustion engine. For example, see Patent Document 1. Patent Document 1 discloses an exhaust system in which two or more collecting tubes joining a plurality of exhaust passages, are connected with a bridge passage serving as a communication tube. The communication tube is adapted to be opened and closed by a valve. Patent Document 1 describes the communication tube as contributing to an improvement in thermal efficiency in an internal combustion engine. Patent Document 1 also describes that the internal diameter of the communication tube may be set to 20 to 100% of the internal diameter of each collecting tube to ensure the contribution to an improvement in thermal efficiency.    Patent Document 1: Japanese Laid-open Patent Publication No. 2001-164934